Antigen recognition by T cells Flashcards
1
Q
Define antigen
A
Proteins, carbohydrates and lipids capable of binding BCRs, TCRs and/ or innate immune receptors to induce an immune response
2
Q
What is an epitope?
A
- Specific portion of an antigen, that a complementary Ab will bind to
- Each antigen will have multiple epitopes that different Abs can bind to = polyclonal response
- Ags will be degraded and processed, and the epitope will be presented on MHC to T cells
3
Q
Why cant T cells recognise native antigens?
A
- Native = not processed
- B-cells can be actiavted if their IgM is cross-linked, allowing proliferation and production of Abs
- Native Antigen has to be processed and presented on MHC before T cells can produce a response
4
Q
What immune cells recognise and process ag?
A
- APCs
- Monocytes = have CD14+ (dont express huge amounts, but they form a reservoir for differentiation into macrophages and DCs)
- Macrophages = CD11b+
- DCs = CD11c+ (myeloid) or CD123+ (plasmacytoid)
- B cells = CD19+ (BCR)
5
Q
What are macrophages and DCs?
A
- Rarely found in peripheral blood - in mucosal tissues
- High phagocytic cells - induce strong T-cell responses and inflammation
- Macrophages = better equipped to kill pathogens (higher NO production)
- DCs = better at migrating to lymph nodes (via CCR7) and presenting ag to T-cells
6
Q
What do B cells do?
A
- Highly abundant in blood and mucosal tissue
- Receptor-mediated internalisation of Ags as opposed to phagocytosis
- Differentiate into plasma cell to make Ab
- Good APCs
- Main inducer of T-cell response against pathogens
7
Q
Fixed vs viable APC
A
- If an APC is viable, it can be presented with native protein, process it and present the ag on MHC to T cells
- If an APC is fixed, it is metabolically inert and so can only present the ag if it has been pre-digested
8
Q
What are the 2 pathways of antigen presenting?
A
- Exogenous - ag taken up from outside the cell. EC bacteria e.g. N.meningitidis or s.aureus
- Endogenous - ag from inside the cell. Viruses e.g. HIV, tumour antigens
9
Q
What are the different processes of exogenous antigen uptake?
A
- B cells use receptor-mediated internalisation
- Phagocytosis - uses PRRs
- Pinocytosis - cell drinking. DCs are constantly sampling the surroundings to detect pathogens
- Fc receptor mediated phagocytosis
- Complement receptor mediated phagocytosis
10
Q
How do cells go from endocytosis to presenting ag?
A
- Exogenous protein is endocytosed and exposed to proteases, NO, low pH and ROS in an endosome
- Will then become a late endosome/lysosome - becomes increasingly toxic
- Fuses with the MHC class II compartment
- MHC class II is exported from the ER and joins the compartment
- Invariant chain sits in the ag receptor for now, stabilising the molecule
- enzymes start to degrade invariant chain, leaving the CLIP peptide, which is then displaced by the ag
- HLA-DM helps to shuttle the loaded MHC to the surface, where it can present to CD4+ cells
11
Q
How does endogenous, cytoplasmic protein get processed?
A
- Cytoplasmic protein gets cleaved after going through the proteasome
- TAP is an ATP dependent transporter that shuttles the peptides into the ER
- Here it is loaded onto MHC I, which like MHC II, is unstable until bound Ag, so needs chaperones
- ER amino peptidases break the particles into smaller pieces in the ER
- When the MHC I is ready and loaded, it is taken up to the surface, through the golgi to the surface, and presented to CD8 T cells
12
Q
What are the differences in processing between endogenous and exogenous ags?
A
Ex
- Ag endocytosed
- Degraded in lysosomes
- Loaded onto MHC class II
- Presented to CD4+ Th cells
End
- Ag already in cell
- Degraded in proteasome
- Loaded onto MHC class I
- Presented to CD8+ CTL cells
13
Q
What happens with pathogens that dont infect APCs?
A
- e.g. avian flu only infects lung ciliated epithelia
- Epithelial cells arent very good at activating the immune system
- Have to carry out ag cross-presentation
14
Q
What is the process of ag cross-presentation?
A
- Endocytosed exogenous ags can go by cytosolic diversion to be fed into the proteasome, allowing MHC I production
- Ags are take to the proteasome, chopped up, fed into the ER and loaded on MHC I
15
Q
MHC class I vs Class II
A
Class I
- Expressed on all nucleated cells
- Binds short peptides
- Presents to CD8+ T cells
- Ags from cytosol + cross presentation
- Has 3 alpha subunits, a beta-2 microglobulin and a cytoplasmic tail
Class II
- Expressed on APCs, thymic epithelia and activated T-cells
- Binds long peptides
- Presents to CD4+ T-cells
- Ags from phagosomes and endosomes
16
Q
What are the 3 signals required for an APC to activate a T-cell?
A
- Ag presented on MHC complex
- Co-stimulatory molecules e.g. CD40L/CD40 or B7/CD28
- Cytokine release to dictate T cell lineage (Th1,2,17 or Treg)
17
Q
What cytokines dictate which T-cell type?
A
Th1 = IL-12
Th2 = IL-4
Th17 = IL-1b, IL-6, TGF-b, IL-23
Treg = IL-2, TGF-b
18
Q
What are the similarities between TCR and BCR?
A
- Belong to Ig superfamily
- Like Fab fragment of antibody
- Large diversity
- High specificity
19
Q
What are the differences in TCR compared to BCR?
A
- Lower affinity
- Cannot be released
- No Fc fragment
- Single rather than two binding sites
- BCR has 5 classes (IgM etc)
- TCR has 2 classes (ab and gd)
20
Q
What are the mechanisms that generate B/T-cell receptor diversity?
A
B cells
- Before Ag stim = somatic recombination
- After Ag stim = somatic hypermutation
T cells
- Before Ag stim = somatic recombination
- After = mone
- receptor gene rearrangement takes place during T-cell development in thymus
21
Q
What is the immunological synapse?
A
- The complex interaction of many molecules between T cell and APC
- MHC-TCR and Co-stimulatory signals are central
- There are lots of surrounding integrins and accessory molecules to helps stabilise the interaction
22
Q
What do the APC-T-cell reactions actually cause?
A
- Peptide-MHC binds to TCR -> Lck, Zap70 and PLC-g –> Calcium flux and NFAT activation -> T cell proliferation and survival
- Co-stimulation with CD28 -> PI3K, Akt, MAPK -> NF-kB activation -> pro-inflammatory survival signals
23
Q
How do T-cells induce changes in contacted cells?
A
- Activated CD8+ T cell can meet a virus-infected cell and secrete enzymes etc, killing the cell
- CD4+Th1 cells will meet macrophage. It will release cytokines such as IFN-gamma and activate the macrophage, allowing it to kill any invading bacteria
- CD4+Th2 cells will interact with a B cell and secrete cytokines that cause class switching. This B cell can differentiate into a plasma cell to secrete abs
24
Q
What are the negative regulators of antigen presentation?
A
- An overly-vigorous response is harmful to the host
- Negative regulators provide an immune checkpoint to limit T-cell activation
- CTLA-4 (cytotoxic-T-lymphocyte-Associate protein 4)
- PD-L1 (programmed death ligand 1)
- Both crucial for dampening the T-cell response
- TCR also down-regulates its own expression once recognised ag
25
Q
How do the negative regulators work?
A
- PD-L1 binds to PD-1, activating SHP-2, which dephosphorylates TCR signalling molecules
- CTLA-4 competes with CD28 for APC attention
26
Q
What are the different T-cell selection processes in the thymus?
A
- tested for self-reactivity
- If binds too strongly = apoptosed because dangerous (negative selection)
- If doesnt bind at all = apoptosed because wont protect against pathogens
- Positively selected if can bind but not too strongly
- In stochastic model, strongly self-reactive CD4+ T-cells can express FOXP3 TF and become Treg cells, and compete with any self-reactive T-cell
27
Q
How can pathogens impede antigen presentation?
A
- TB = upregulates PD-L1 and blocks MHC II expression
- n.meningitidis = blocks DC activation by downregulating receptors and MHC. Has capsule
- n.gonorrhoeae = expresse Opa protein, binds to T cells and induces tyrosine phosphatases, switching ogg TCR signalling molecules
- HIV - upregulates PD-L1 on T-cells and suppresses DC activation
- HSV = produce protein which inhibits TAP, prevents viral peptide transfer to ER
- Adenovirus = produce protein that binds to MHC class I preventing it from leaving ER
